Described herein is a magnetic recording medium comprising a substrate and a magnetic recording layer thereon comprising a cured binder and magnetic particles, the binder comprising a lactone-acrylate adduct.
Magnetic recording media in the form of magnetic cards and disks, reels, video tapes, computer tapes etc. are generally prepared by coating a non-magnetizable backing member, such as polyethylene terephthalate film, with a coating of magnetizable particles in a binder. The binder can be made from a variety of thermoplastic and thermosetting resins. For example, in U.S. Pat. No. 4,059,715 there is described, as a binder, a thermoplastic copolyester consisting of 5 to 75 percent by weight of amorphous ester units and 95 to 25 percent by weight of crystallizable ester units joined through the ester linkages. U.K. Patent Application No. GB 2085906, published May 6, 1982, describes a particular thermosetting resin as a binder, which is stated to be preferred because of its favorable effects upon the physical properties such as resistance to wear, heat and solvents of the final magnetic recording medium. In the U.K. Patent Application, the magnetic recording medium is made by using: (A) a compound having two or more radiation-curable, unsaturated double bonds and a molecular weight of not less than 5000, (B) a compound having one or more radiation curable, unsaturated double bonds and a molecular weight of over 400 and less than 5000, and (C) a compound having one or more radiation-curable, unsaturated double bonds and a molecular weight of less than 400. The substrate is then coated with a magnetic material, stated in the U.K. Patent Application to be composed essentially of a binder which is a mixture of the compounds (A) and (B), or of (A), (B), and (C), magnetic particles, and a solvent. The coating is stated to be predryed and then the coated matter is irradiated with electromagnetic rays. The compound (A) is one prepared by reacting either a partly saponified vinyl chloride-vinyl acetate copolymer or a vinyl chloride-vinyl alcohol copolymer, obtained by saponifying almost all of the acetyl groups of the vinyl chloride-vinyl acetate copolymer, with a polyisocyanate compound, and then reacting the resulting isocyanate-containing compound with an acrylic or methacrylic compound having a functional group reactive with the isocyanate group. The compound (B) is one prepared by reacting a polyol with a polyisocyanate compound and then reacting the resulting isocyanate-containing compound with an acrylic or methacrylic compound having a functional group reactive with the isocyanate group. The compound (C) is described (on page 4) of the U.K. Patent Application as styrene, ethylacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol dimethacrylate, 1,5-hexane glycol diacrylate, 1,6-hexane glycol dimethacrylate, trimethylolpropane triacrylate, and trimethylolpropane trimethacrylate.
The U.K. Patent Application states that the above-mentioned compounds (A), (B), and (C), when adopted singly, will provide radiation curable coatings but will not always give a magnetic coating material of good physical properties. It is by mixing the compounds (A), (B), and (C) of different molecular weights that curability is stated to be attained even with a low dosage of about 1-3 mrads, and many different physical property requirements can be met. Especially, the binders consisting of the (A)-(B) and (A)-(B)-(C) combinations are stated to be superior to the binder of the (A)-(C) combination in bonding ability, flexibility, etc.
In the present invention it has been found that a select group of materials can be used as binders in magnetic recording materials. The use of these binders results in a magnetic recording material which has an excellent balance of mechanical properties.